CURVATURE OF STRATA 143 
crust yields unequally to the lateral compression induced by its sub- 
sidence on the cooling and contracting interior. The younger mountain 
ranges of the globe—the Alps, the Himalayas, the Andes, the Rockies, 
etc.—are composed essentially of highly disturbed and complexly folded 
and contorted rocks, and are believed, therefore, to indicate zones of 
weakness, along which relief from pressure has been readily obtained. 
Similarly, much more ancient zones of steeply flexed and folded rocks, 
occurring, it may be, in low-lying regions, show geologists where gigantic 
mountains formerly existed, and assure them that from the earliest times 
the crust has found relief from lateral pressure by buckling up along lines 
of weakness. 
Some further remarks on folding and its results will be found in 
the following chapter. 

